Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching which have been developed for the fabrication of integrated circuits.
A digital micromirror device (DMD™), sometimes referred to as a deformable micromirror device, is a type of micromechanical device. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors. While some micromechanical devices, such as pressure sensors, flow sensors, and DMDs have found commercial success, other types have not yet been commercially viable.
Digital micromirror devices are primarily used in optical display systems. In display systems, the DMD is a light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, DMDs typically operate in a digital bistable mode of operation and as such are the core of the first true digital full-color image projection systems.
Micromirrors have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane which, when electrostatically attracted to an underlying address electrode, dimpled toward the address electrode. Schlieren optics illuminated the membrane and created an image from the light scattered by the dimpled portions of the membrane. Schlieren systems enabled the membrane devices to form images, but the images formed were very dim and had low contrast ratios, making them unsuitable for most image display applications.
Later micromirror devices used flaps or diving board-shaped cantilever beams of silicon or aluminum, coupled with dark-field optics to create images having improved contrast ratios. Flap and cantilever beam devices typically used a single metal layer to form the top reflective layer of the device. This single metal layer tended to deform over a large region, however, which scattered light impinging on the deformed portion. Torsion beam devices use a thin metal layer to form a torsion beam, which is referred to as a hinge, and a thicker metal layer to form a rigid member, or beam, typically having a mirror-like surface: concentrating the deformation on a relatively small portion of the DMD surface. The rigid mirror remains flat while the hinges deform, minimizing the amount of light scattered by the device and improving the contrast ratio of the device.
Recent micromirror configurations, called hidden-hinge designs, further improve the image contrast ratio by fabricating the mirror on a pedestal above the torsion beams. The elevated mirror covers the torsion beams, torsion beam supports, and a rigid yoke connecting the torsion beams and mirror support, further improving the contrast ratio of images produced by the device.
Micromirror devices have proven very difficult to manufacture. Not only are the steps of forming the mirrors difficult to perform in a production environment, the completed device is extremely sensitive to debris generated by the production process. While most semiconductor devices can be washed to remove debris and contaminants, the surface tension of a liquid used to wash the micromirror device destroys the mirror array. Therefore, extreme caution must be used to avoid creating debris once the mirrors are fully formed and the sacrificial layers on which they were formed have been removed.
One process that causes failures is the package bond out process. Once the completed device has been attached to the device package, bond wires are added between bond pads on the integrated circuit and bond pads in the package. A reliable electrical ground between the package and the bond machine is necessary for the wire bonder to electrically test the connection between the gold bond wire and the integrated circuit or package. A very small electrical current is applied to verify the electrical connection through a very large resistance path to ground. It is critical that a good ground is maintained between the package ground and the wire bonder. If the ground fails, the wirebond monitoring system (WBMS) will sense an open circuit between the bond wire and the ground and assume that the failure is due to a poor connection between the bond wire and the bond pad when in fact the bond wire connection may be good.
Prior art mechanisms used a clamp pressed against the seal ring at the top of the DMD package to ensure and adequate ground. While the clamp established and maintained a good ground between the wire bonder and the device package, it also generated debris particles from contact between the package and the clamp. Since the contact was on the top of the package near the device, the particles generated could easily contaminate the mirror array. What is needed is a method of holding the package in place and establishing a reliable ground connection without generating debris that can enter and damage the mirror array.